The present invention relates to articles formed from polypropylene compositions suitable for metal plating. The plated articles exhibit greatly improved bonding between the metal coating and the surface of the polypropylene.
Those engaged in the manufacture of metallic articles, particularly decorative, and other non-load bearing metallic articles, have long desired to reduce the frequently inordinate expense of manufacturing such articles. One of the most obvious possibilities for effecting such reductions in costs, would be to substitute a lighter and/or less expensive material for a large portion of the metal in relatively heavy articles. Another obvious possibility would be to simplify the extremely expensive molding, shaping and polishing procedures which must usually be employed in manufacturing such an article from metal. Quite some time ago, it was observed that it might be possible to obtain both of the aforementioned savings by molding or shaping the article from a plastic material, and subsequently plating the surface of the molded plastic article with a coating of metal.
There are of course a number of criteria to be met in order to produce such articles by molding from a plastic and subsequently plating with a metal. To name but a few, the physical property characteristics of the plastic must usually be sufficient to meet all the specifications required of the final article; these physical properties must not be impaired by degradation of the plastic due to contact with the metal; the plastic must be readily platable by vacuum deposition techniques, chemical deposition techniques, electrodeposition techniques, and/or a combination of such techniques, to produce a continuous, uniform metal coating covering 100 percent of the area to be plated; and, the coating of metal must adhere firmly to the surface of the molded plastic article.
Specific applications in which manufacturers have sought to substitute a platable plastic composition for all or part of the former metal content are both numerous and diverse. Recently, for example, a number of possible applications for platable plastic materials for the automotive industry have been suggested, including fabrication of grills and hubcaps. Like most other proposed applications for platable polymers, fabrication of automotive grills and hubcaps would involve the use of significant quantities of materials to produce a very detailed decorative configuration, having a plurality of geometric planes forming essentially a single surface. The use of platable polymer in such applications is not only advantageous from the point of view of substituting a lighter or less expensive plastic for much of the metal which would otherwise be required, but also because of the efficiency and effectiveness with which extremely detailed and/or structurally shaped configurations can be achieved with plastic molding equipment.
The phenomenal increase in the commercial significance of polypropylene is in many cases attributable to the very low specific gravity of unfilled polypropylene. A substantial reduction in the weight of polymer required for a given application can often be achieved by substituting polypropylene for other popular thermoplastic polymers. Its low specific gravity makes polypropylene a particularly desirable polymer for fabrication of large metal plated plastic articles, such as those described above. Unfortunately, unfilled polypropylene compositions heretofore commercially available have had certain inherent property characteristics which seriously limited their use in manufacturing certain types of articles, including automotive applications such as those noted above. In particular, it is well known that in the absence of large amounts of filler, polypropylene compositions are generally subject to volumetric change with temperature. Shrinkage of the polymer composition during cooling of the mold, particularly in areas where there was a change of cross-section in the molded article, resulted in so-called "sink marks," or other surface imperfections and deformities. While it is often possible to eliminate these problems by incorporating from about 20 to about 30 part of filler (per 100 parts of resin) into the polypropylene composition, this may destroy the specific gravity advantage which may originally have made polypropylene desirable.
The general procedure for manufacturing shaped articles by metal plating an article which has been pre-formed from a polymeric material, comprises the steps of:
A. forming the polymeric material into the desired shape;
B. cleaning and treating the surface of the formed article to render it more receptive to the metal coating;
C. depositing a metal coating by chemical or vacuum deposition techniques onto the surface of the article; and, if desired, subsequently
D. electroplating the coated article to produce the desired deposit of metal coating.
It will of course be understood that each of the steps of the foregoing general procedure can involve a series of specific sub-steps, which are in any given instance a function of the polymer to be employed, the specific metal to be chemically or vacuum deposited onto the surface of the plastic to render it conductive, and the specific metal or metals to be electroplated onto the surface after it has been rendered conductive.
The cleaning or surface treatment steps generally involve washing or immersion in a de-greasing and cleaning solution to remove both surface dirt and any traces of polymer additives which may have migrated to the surface, particularly mold release agents or the like. Following the cleaning step the articles are usually immersed in an acid etching solution, and/or the surface of the shaped plastic article is otherwise treated to improve adhesion of the subsequently deposited metal coating.
In an effort to develop polymeric compositions for plating applications, a number of thermoplastic and thermosetting polymers have been extensively tested, particularly during the past ten years. For example Belgian Patent 613,430 (Feb. 28, 1962) mentions metal plating of a wide variety of polymers, including, polyolefin polymers, vinyl polymers, polyamides, polyurethanes, polystyrenes, polyacrylates, and the like. Various other types of polymers, particularly acrylonitrile-butadiene-styrene (ABS), have also been the subject of extensive experimentation, and even some limited commercial use.
The interest in developing commercial platable plastic compositions, and/or techniques, have also led to the investigation of a wide range of special additives and procedures. For example, U.S. Pat. No. 3,466,232, to Peter S. Francis et al., discloses and claims a method of plating plastic materials in which improved adhesion is obtained by incorporating into the plastic material an additive which will be etched out during the surface preparation step, thereby producing a large number of small pockets having openings in the surface which are smaller in diameter than the largest diameter through their respective pockets.
In effect, the Francis et al. procedure provides a series of undercut pores which will be filled with metal during the plating steps, thereby serving as anchoring points for the metal coating. While the procedure disclosed by Francis et al. does provide a method of improving adhesions, the concentration of additives used in the examples to obtain optimum bondings, was in the order of 33 parts per hundred of resin, a level which can have a serious deleterious effect on the basic physical properties of the polymer.
In the area of special additives, South African Patent Applications, Serial No. 68/2279 and Ser. No. 68/2282 were published under the title "Electroplatable Polyolefins." Both of these applications claim priority of United States applications filed May 3, 1967, (which issued Mar. 10, 1970 as U.S. Pat. No. 3,499,881) and both relate to polyolefin compositions exhibiting improved adhesion to plated metal coatings.
Application Ser. No. 68/2279 discloses polyolefin compositions containing from 0.25 to 1.5 parts by weight, per hundred parts by weight of resin (hereinafter "phr" for convenience) of a compatible sulfur compound, such as dilauryl thiodipropionate, and from 10 to 60 phr of a sulfate of a metal of Group II of the Period Table of Elements, such as barium sulfate. Suitable polymers, according to the specification, are polyethylene, polypropylene, polybutene-1, poly(4-methylpentene-1), poly(3-methylbutene-1), ethylene propylene copolymers, ethylene butene-1 copolymers, and the like.
Application Serial No. 68/2282 discloses polyolefin compositions containing from 0.1 to 2.0 phr of a compatible surfactant (highly polar and ionic surfactants being specifically excluded), and in the preferred compositions, from 0.25 to 1.5 phr of a compatible sulfur compound such as dilauryl thiodipropionate. Suitable polyolefin polymers according to this specification include, in addition to those disclosed in application Ser. No. 68/2279, vinyl aromatic polymers and predominantly olefinic copolymers of hydrocarbon monomers with copolymerizable polar monomers such as acrylic monomers and vinyl esters.
The compositions of the above noted South African patent application, as well as the novel procedure of Francis et al., generally provide improved adhesions in metal plated plastic articles. In fact, for some time now certain limited commercial success has been reported in producing metal plated articles from ABS compositions, and more recently from polypropylene. In spite of this limited commercial success however, and even with the recent improvements described above, a generally satisfactory commercial polymer composition and complimentary procedure, have not heretofore been found.
Other types of investigations have also been made in the area of platable plastic compositions, investigating the effect on adhesion of such things as, the types and amounts of filler variations in molding equipment used in forming the plastic, and/or variations in the operating conditions or a given piece of equipment, e.g., faster cooling, slower cooling, higher or lower temperatures, and the like. Of particular interest is the recent work of Fitchmum et al., "Surface Morphology in Semi-Crystalline Polymers," Polymer Letters, Vol. 7, pp. 301-305 (1969). The authors indicate that the surface against which the polymer is molded, e.g., aluminum, copper, Mylar, Teflon, etc., apparently has an effect on the surface characteristics of the molded part, which in turn appears to affect the adhesion of the metal coating which is subsequently plated onto the surface. Fitchmun et al. theorize that the surface crystallinity is altered and that the surface crystallinity may therefore affect adhesion.
The theory of Fitchmun et al, that the changes in adhesion were directly related to an alteration of surface crystallinity, will undoubtedly be the subject of further work by both the original investigators and others. At this time, based on the results we have observed in our testing, we are not in a position to form an opinion one way or the other, with respect to the surface crystallinity theory. Nevertheless, the work of Tichumun et al. would seem to have clearly established that adhesion is, to at least some extent, a function of the surface of the molded plastic article. As will be seen in more detail hereinafter, our tests confirm the observations of Fitchmun, that there is a direct relationship between the surface of the molded article, and the adhesion to that surface of a subsequently plated metal coating.
The evaluation of the adhesion between the metal coating and a molded plastic substrate, is generally based on the pounds of mechanical force required to peel the metal coating from the substrate, in a 90.degree. peel test with a tensometer. In general, the testing procedure is referred to as the "Jacquet Test," see for example, P. A. Jacquet, Transactions of the Electrochemical Society, Vol. 66, p. 393 (1934). A number of modifications or variations of the Jacquet Test have also been used, and several have been published, including Saubestre et al., "The Adhesion of Electrodeposits to Plastics," presented at the American Electroplaters Soceity 52nd Annual Convention, July 13, 1965; and also "Testing of Plating on Plastics," Product Finishing, Vol. 18, No. 5, May, 1965. Nevertheless, the essential nature of the test remains the same, the plated sample is prepared, and a cut is made through the metal to form one or more strips of a given width, a portion of the test strip is peeled part way back, the plated test sample piece is clamped horizontally in the fixed jaw of the tensometer, and the peeled portion of the metal strip is clamped vertically in the pulling jaw of the tensometer. The pulling jaw is then retracted at a constant rate of separation and the tensile load, acting at approximately 90.degree., is measured.